Safety glass has existed for more than 80 years, and is widely used for windows in trains, planes, ships, etc. and in the automotive industry, for example, in windshields for cars, trucks and other forms of transportation. It is characterized by high impact and penetration resistance and it does not scatter glass shards and debris when shattered. Safety glass is also used in the construction industry and in the design of modern buildings. It is used, for example, as windows for stores and offices.
Safety glass usually consists of a sandwich of two glass sheets or panels bonded together by means of an interlayer of a polymer film placed between the two glass sheets. One or both of the glass sheets may be replaced by optically clear rigid polymer sheets, such as sheets of a polycarbonate polymer.
The interlayer is made of a relatively thick polymer film exhibiting a toughness and bondability as will cause the glass to adhere to the interlayer in the event of its being cracked or crashed. A number of polymers and polymer compositions have been used to produce transparent interlayer films for bilayer and multiple layer mineral or polymer glass sheets.
Polymer interlayers for mineral and plastic glass must possess a combination of characteristics including very high clarity (low haze), high impact and penetration resistance, excellent UV-light stability, good bondability to glass, low UV-light transmittance, low moisture absorption, high moisture resistance, and extremely high weatherability. Widely used interlayers in safety glass production today are made of complex multicomponent formulations based on polyvinyl butyral (PVB), polyurethane (PU), polyvinylchloride (PVC), ethylene copolymers such as ethylenevinylacetate (EVA), polymeric fatty acid polyamide (PAM), polyester resins such as polyethyleneterephthalate (PET), silicone elastomers (SEL), epoxy resins (ER) or polycarbonates (PC) such as elastomeric polycarbonates (EPC).
Many major glass laminate manufacturers are of the opinion that PVB compositions provide the best overall performance taking costs into consideration. These PVB compositions, therefore, have become the interlayer of choice for many laminated glass applications. Although conventional PVB interlayers perform well, they do, nevertheless, suffer from several drawbacks.
One major drawback of PVB is its moisture sensitivity. Increased moisture in interlayer films results in increased haze and may cause bubble formation in the final laminated flat glass product. This is a problem particularly around the edges of laminates and the extent of the problem increases markedly over time. This is unacceptable to both the manufacturers and their customers. Therefore, special precautions have to be taken to keep the moisture content of the PVB film, and ultimately the haze of the laminated flat glass product, to a minimum. These special precautions may include reducing storage time of the PVB film; refrigeration of the PVB film prior to lamination; pre-drying of the PVB film; and/or using dehumidifiers in the clean rooms where the laminates are prepared. These requisite precautions increase the cost and the difficulty of manufacturing laminates made with a polyvinyl butyral interlayer. Furthermore, despite these precautions and added manufacturing costs, when the edges of the laminated glass are exposed to moisture, a haze will still develop. This becomes a serious problem with the modern flush-mounted windshield favored by modern car designers. These designs call for far less overlap of the rubber mounting holding the laminate in the window aperture. To conceal any haze formation that may develop over time, manufacturers have taken to printing a pattern of black dots, the density of which decreases with distance from the edge of the laminate, around all of the edges.
Another drawback of PVB is the need for a plasticizer in the film formulation for improving the impact, tear and penetration resistance and for improving the bonding of the PVB to the glass. Over time, the plasticizer tends to migrate, leading to changes in the properties of the laminate. One particular concern is that delamination will begin to occur at the edges of the laminated glass and the interlayer will become brittle and lose its safety features.
A very significant drawback of PVB film and optical laminates made using PVB film is the low impact resistance at low temperatures due to the very high glass transition temperature (Tg)of PVB which is close to room temperature 21xc2x0 C. (70xc2x0 F.) The Tg of plasticized formulations is in the range from 0xc2x0 C. to minus 10xc2x0 C. At temperatures below zero the safety glass made using PVB can be relatively easily destroyed by impacting, and may lose its safety properties.
While many of the other polymers and formulations do not have a moisture absorption problem as significant as PVB or Surlyn(trademark) resin (a Dupont ionomeric resin), they lack the overall performance of the PVB films at comparable costs. Furthermore, some of these polymers and formulations require enhanced processing such as irradiation or the use of additional chemical components such as plasticizers which affect the cost and properties of the film and the optical laminates, e.g., flat glass products, made using the film. Plasticizers tend to migrate over time. This adversely affects the properties of both the film and the products made using the film.
Recently developed metallocene catalyzed, linear low density polyethylene (LLDPE) having very low heat seal temperature, low extractables and improved clarity (compared to LLDPE polymerized using conventional and modified Ziegler-Natta catalysts) has been designed for packaging applications. For example, a metallocene LLDPE film which exhibits a density of at least 0.900 g/ccm, low heat seal temperature, low extractables, and a haze value of less than 20%, is disclosed in U.S. Pat. No. 5,420,220. Packaging film according to this disclosure has less haze when compared to a film extruded of a conventional Ziegler-Natta LLDPE (exhibiting typical haze values greater than 10%). However, haze was measured by ASTM method D-1003 for very thin film samples (0.8-1.0 mil, or approx. 20-25 mcm). Films of much higher thickness (7-14 mil) are used for optical laminates, and the disclosed packaging film is not able to provide the required optical properties. For example safety glass products have to exhibit a haze lower than 4%, some of them lower than 2 or 1%, and in the most demanding car windshields applications 0.3-0.5%, for thicknesses in the range from 5 mil to 40 mil.
It has now been discovered that an economical, easily processed optical laminate with improved properties may be fabricated from polymer glass and/or mineral safety glass containing an interlayer film made of a formulation based on a substantially linear very low or ultra-low density polyethylenic polymer, copolymer, or terpolymer, their blends and alloys. In modern industry the term linear low density polyethylene (LLDPE) relates to an ethylenic polymer or copolymer having a density from 0.925 g/ccm to 0.910 g/ccm; the term linear very low density polyethylene (LVLDPE)xe2x80x94from 0.910 g/ccm to 0.880 g/ccm; and the term linear ultra-low density polyethylene (LULDPE)xe2x80x94from 0.880 g/ccm to 0.850 g/ccm.
Very low and ultra-low density polyethylene and their copolymers with butene, octene, hexene, propylene, pentene, and other comonomers are produced using various metallocene catalyst systems. The substantially linear, very low and ultra-low density ethylenic polymers and copolymers, when co-extruded or laminated with one or more layers of EVA film, provide an interlayer film for a glass xe2x80x9csandwichxe2x80x9d characterized by excellent adhesion to glass and polymeric substrates and having high clarity, very high moisture resistance, extremely low moisture absorption during storage, handling and use, very high UV-light stability, and good heat resistance. Low density, high yield (a higher number of square meters of film produced from one weight unit of resin) and higher impact and penetration resistance of these polymers enables one to use a thinner interlayer film and provides significant economical advantages compared to PVB and monolithic EVA films and their optical laminates. The costs of suggested interlayer can be 20-30% less than conventional PVB interlayer. The cost of an interlayer is usually about 30% of the cost of the final optical laminate. Therefore the significant cost reduction of the interlayer translates into substantial cost reduction of the laminated glass product.
The invention is an interlayer film for use in an optical laminate, the optical laminate having a haze value of less than 4% at a film thickness of 125 to 1000 mcm, comprising a film of at least one substantially linear low density polyolefin, having a polydispersity index of less than 3.5, a density from about 0.850 to 0.905 g/ccm and less than 20%, by weight, crystallinity, laminated to at least one film of ethylene vinyl acetate copolymer. The invention includes laminates made with the interlayer film.
This invention also provides optical laminates containing the interlayer film and a process for manufacturing these products comprising the steps of selecting a metallocene catalyzed substantially linear, very low density polyethylene (LVLDPE) having a density lower than about 0.905 g/ccm or ultra-low density polyethelyne (LULDPE) having a density lower than about 0.880 g/ccm, Laminating a 125 to 1000 mcm thick film of the selected polyethylene to at least one film of EVA to form an interlayer film, and incorporating this interlayer film between at least two sheets of mineral or polymer glass. It is understood that the terms xe2x80x9cLVLDPExe2x80x9d and xe2x80x9cLULDPExe2x80x9d as used herein embrace not only homopolyethylene but also copolymers of ethylene with other comonomers known in the art, such as alpha olefins (e.g., butene, octene, propylene, pentene and hexene).
The interlayer film may also comprise an additive package consisting of: coupling agents (0.1 to 2.0%, by weight) to improve adhesion to glass and/or plastic panels; clarifying (nucleation) agents (0.02 to 2.0%, by weight) to increase the light transmittance (reduce the haze) of the interlayer; and UV-light absorbers to decrease the UV-light transmittance. Other additives also can be incorporated to achieve special properties in the safety glass and/or plastic laminates. A crosslinking agent may be added in the amount of from about 0.05% to about 2% by weight of the total formulation. Examples of other additives include pigments, colorizing agents or concentrates and IR-light blockers.
The films of the present invention also may be used as an interlayer in other multilayer products manufactured using mineral glass or plastic sheets or panels.